1, determine the number of layers of the PCB
The number of board sizes and wiring layers need to be determined in the early stage of the design. If the design requires the use of a high-density ball grid array (BGA) component, you must consider the number of small line layers required for these device wirings. The number of wiring layers and the stack-up mode will directly affect the wiring and impedance of the printing line. The size of the board helps to determine the laminated mode and print wire width, and achieve the desired design effect.
Over the years, people always believe that the lower the number of circuit boards, the lower the cost, but there are still many other factors that affect the manufacturing cost of the board. In recent years, the cost difference between multi-layer board has been greatly reduced. There is a large number of circuit layers when the design is started, and the copper is uniformly distributed to avoid that there is a small number of signals that do not meet the defined rules and space requirements at the end of the design, thereby being forced to add a new layer. The serious plan before the design will reduce a lot of trouble in the wiring.
2, design rules and restrictions
The automatic wiring tool itself does not know what should be done. To complete the wiring task, the wiring tool needs to work under the correct rules and restrictions. Different signal lines have different wiring requirements, and to classify all special requirements, different design classifications are different. Each signal class should have priority, the higher the priority, the more stringent rules. Rules relate to printed wire width, via, parallelism, signal line interaction between the line, and the limitations of the layer, which have a great impact on the performance of the wiring tool. Seriously considering design requirements is an important step in successful wiring.
3, the layout of components
For assembly processes, the manufacturability design (DFM) rules will restrict the assembly layout. If the assembly department allows the component to move, it can be properly optimized to the circuit, which is more convenient for automatic wiring. The defined rules and constraints will affect the layout design.
The RoutingChannel and the via region are considered in the layout. These paths and regions are obvious for designers, but the automatic wiring tool will only consider a signal, and the wiring tool can make the wiring tool like the designer as designer by setting the wiring constraint condition and the layers of the displayable signal line. Complete the wiring.
4, fan out design
In the fan-out design phase, to connect the automatic wiring tool to the component pin, each pin of the surface mount device should have a via hole in order to be more connected, so that the circuit board can perform the inner layer when needed Connection, online test (ICT) and circuit processes.
In order to make the automatic wiring tool efficiency, it is necessary to use the via size and printed lines as much as possible, and the interval is set to 50 mil is ideal. To use a via type that makes the wiring path. When performing fan-out design, consider the circuit online test problem. Test clamps may be expensive, and usually order when they are about to be put into full production, if you consider adding nodes at this time, it is too late to implement 100% testability.
After careful consideration and prediction, the design of the circuit online test can be carried out in the initial design of the design, and it is achieved in the latter path and circuit online test, and the power supply and ground can also affect the wiring and fan out of the design according to the wiring path and circuit online test. . In order to reduce the induction generated by the filter capacitor cable, the via should be as close as possible to the pin of the surface mounting device, if necessary, can use the manual wiring, which may have an impact on the originally envisaged wiring path, and may even lead you to re- Which via is used to use, so you must consider the relationship between the via and the inductance, the priority of the via specifications must be considered.
5, manual wiring and key signal processing
Although this paper focuses on automatic wiring problems, manual wiring is an important process of designing printed circuit boards in the present and future. The manual wiring will help the automatic wiring tool complete the wiring work. As shown in FIGS. 2A and 2B, by performing a manual wiring and fixing the selected network (NET), it can be based on the path when the automatic wiring can be formed.
Regardless of the number of critical signals, it is first possible to wiring, manually wiring or binding automatic wiring tools. Key signals typically have to achieve desired performance through careful circuit design. After the wiring is completed, these signal wirings are checked by the relevant engineers, which is relatively easy. After checking, secure these lines and then start automatic wiring for the remaining signals.
6, automatic wiring
The wiring of the key signal needs to be considered to control some of the electrical parameters when the wiring, such as reducing distribution inductance and EMC, etc., the wiring of other signals is similar. All EDA vendors provide a way to control these parameters. After understanding the input parameters of the automatic wiring tool and the input parameters affect the wiring, the quality of the automatic wiring can be guaranteed to a certain extent.
The general rules should be used to automatically wiring the signal. By setting the restriction condition and the prohibition of the wiring zone to define the layer used by the given signal and the number of vias used, the wiring tool can be routed according to the engineer's design idea. If the number of layers and the number of the cloth used for the automatic wiring tool are not limited, each layer will be used when the automatic wiring is used, and there will be many via holes.
After setting the rules created by the constraints and applications, the automatic wiring will reach the results similar to expected, of course, some sorting work may also be required, and there is also a space for other signals and network wiring. After a portion of the design is completed, it is fixed to prevent the effects of the rear wiring process.
Wiring the remaining signals using the same step. The number of wirings depends on the complexity of the circuit and how much you define. After a class of signals are completed, the constraints of the remaining network wirings are reduced. However, it is that many signal wirings need to be manually intervened. The current automatic wiring tool is very powerful, usually 100% wiring. However, when the automatic wiring tool does not complete all the signal wirings, the remaining signal is required to make a manual wiring.
7, the design points of the automatic wiring include:
1) Slightly change the setting, trial multiple path wirings;
2) Maintaining the basic rules unchanged, trial different wiring layers, different printing lines and spacing widths, and different line width, different types of via holes such as blind holes, buried holes, etc., what is the impact of these factors for design results;
3) Let the wiring tool to process those default networks as needed;
4) The less the signal is, the larger the degree of freedom of the automatic wiring tool.
8, the finishing of the wiring
If the EDA tool software you use can list the wiring length of the signal, check the data, you may find that some signal wirings of some constraints have a long length. This problem is relatively easy to handle, shortening the length of the signal wiring and reducing the number of overfores by manually editing. During the finishing process, you need to judge which wiring is reasonable, which wiring is unreasonable. Like the manual wiring design, automatic wiring design can also be organized and edited during the inspection process.
9, the appearance of the board
Previous designs often pay attention to the visual effect of the board, now different. Automatic design circuit boards are not better than manual design, but can meet the requirements of the requirements in electronic properties, and the complete performance of the design is guaranteed.
Reprinted from Verid Electronic Market Network.
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